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For the shortcrust pastry:
1) In a large mixing bowl stir together the flour, sugar and salt. Cut in the butter and vegetable shortening with a pastry cutter or a fork until the mixture resembles coarse breadcrumbs. Work in the ice water 1 tablespoon at a time. If the pastry still does not come together add more ice water as needed. Wrap the pastry tightly in cling film and chill for at least 30 minutes.

2) Roll the pastry out on a floured surface to a 25cm circle. Transfer the pastry to a pie tin and press it down into the bottom and sides. Fold the edges under and flute or crimp them with a fork.

For the filling:
1) Preheat the oven to 190C/Gas 5. In a mixing bowl cut together the flour, sugar, brown sugar, shortening and butter until the crumble resembles coarse breadcrumbs; set aside.

2) In another mixing bowl whisk together the egg, treacle, hot water and bicarbonate of soda. Pour the egg mixture into the pastry shell. Sprinkle the crumble mixture on top.

3) Bake the pie for 30 to 35 minutes. The centre may still quiver a bit when jiggled; it will firm up a bit more as it cools. Serve cold.